Cracks in concrete-block walls

Q. We have a 1950s-built home on a sandy lot where most of the homes have cement-block walls and many have had to be reinforced. Our walls have some cracks, mostly along a horizontal seam. We’ve lived here for seven years and the cracks do not appear to be growing. Can we fill those cracks with some sort of filler?

We’re concerned about resale and assume this issue needs to be addressed. We did have a basement specialist visit, who said the situation was dire and tried to sell us a very expensive fix. We’re doubtful, since the cracks are not growing, that we need to do this. Thoughts? Thank you.

A. The cracks were caused by frost pressure at some time in the past. This is a common problem. I have seen similar cases in houses built on sandy soils.

The sand may be very fine and, as a result, does not drain as fast as a coarser material does.

In the fall, following a significant rainfall, the temperature may have dropped very fast to below freezing, which caused the saturated sand to freeze and expand, and apply enough pressure on the block walls to push them in.

These cracks usually happen about three or four courses below grade, and as long as they are not more than a half inch wide, and no movement has occurred in the seven years you have lived in the house, there seems to be no reason to be overly concerned. The walls seem to have stabilized and there should be no need to do anything to them.
But if they are wider than 1/2-inch, consult a structural engineer for his or her opinion on the need to perform repairs.

However, keep in mind that another set of similar conditions – rain followed by a sudden deep freeze — is always a possibility, and it may aggravate the existing situation.

My guess is that your block walls are only 8 inches thick. Had they been built with 12-inch blocks, as required by building codes for foundations deeper than 5 feet, they might have resisted the mild frost pressure, which yours encountered.

This condition is seldom seen on concrete walls, although I have seen such walls with significant cracks, but they were in houses built in heavy soils and backfilled with the native soil instead of coarse material.

It is often best to leave the cracks alone in the early years following their occurrence as the walls may straighten up when the sandy soil dries up if this does not result in the sand settling down. But since no movement has occurred in seven years, you may want to have a mason fill the joints in for aesthetic reasons.

Even if you get the joints filled with mortar, it may still raise questions at the time of a sale. But an experienced home inspector should be able to assess the severity of the cracks and reassure prospective buyers if need be.

You didn’t say what the basement specialist offered as a fix. There are several ways to repair cracked block walls from simply stabilizing them to bringing them back to their original form. The best fix, if one is indeed needed, is to fasten special fiberglass strips vertically several feet apart on the block walls. An engineer is best to specify the spacing between these strips.